Lithographic apparatus

ABSTRACT

A barrier member is provided for use in immersion lithography. The barrier member includes an extractor assembly on a bottom surface configured to face the substrate. The extractor assembly includes a plate configured to split the space between a liquid removal device and the substrate in two such that a meniscus is formed in an upper channel between the liquid removal device and the plate and below the plate between the plate and the substrate.

RELATED APPLICATIONS

This application is a continuation-in-part of co-pending U.S. patent application Ser. No. 11/274,888 filed Nov. 16, 2005, the entire contents of which is hereby incorporated by reference.

FIELD

The present invention relates to a lithographic apparatus.

BACKGROUND

A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.

It has been proposed to immerse the substrate in the lithographic projection apparatus in a liquid having a relatively high refractive index, e.g. water, so as to fill a space enable imaging of smaller features since the exposure radiation will have a shorter wavelength in the liquid. (The effect of the liquid may also be regarded as increasing the effective NA of the system and also increasing the depth of focus.) Other immersion liquids have been proposed, including water with solid particles (e.g. quartz) suspended therein.

However, submersing the substrate or substrate and substrate table in a bath of liquid (see, for example, U.S. Pat. No. 4,509,852 means that there is a large body of liquid that must be accelerated during a scanning exposure. This requires additional or more powerful motors and turbulence in the liquid may lead to undesirable and unpredictable effects.

One of the solutions proposed is for a liquid supply system to provide liquid on only a localized area of the substrate and in between the final element of the projection system and the substrate using a liquid confinement system (the substrate generally has a larger surface area than the final element of the projection system). One way which has been proposed to arrange for this is disclosed in PCT patent application publication WO 99/49504. As illustrated in FIGS. 2 and 3, liquid is supplied by at least one inlet IN onto the substrate, preferably along the direction of movement of the substrate relative to the final element, and is removed by at least one outlet OUT after having passed under the projection system. That is, as the substrate is scanned beneath the element in a −X direction, liquid is supplied at the +X side of the element and taken up at the −X side. FIG. 2 shows the arrangement schematically in which liquid is supplied via inlet IN and is taken up on the other side of the element by outlet OUT which is connected to a low pressure source. In the illustration of FIG. 2 the liquid is supplied along the direction of movement of the substrate relative to the final element, though this does not need to be the case. Various orientations and numbers of in- and out-lets positioned around the final element are possible, one example is illustrated in FIG. 3 in which four sets of an inlet with an outlet on either side are provided in a regular pattern around the final element.

A further immersion lithography solution with a localized liquid supply system is shown in FIG. 4. Liquid is supplied by two groove inlets IN on either side of the projection system PL and is removed by a plurality of discrete outlets OUT arranged radially outwardly of the inlets IN. The inlets IN and OUT can be arranged in a plate with a hole in its center and through which the projection beam is projected. Liquid is supplied by one groove inlet IN on one side of the projection system PL and removed by a plurality of discrete outlets OUT on the other side of the projection system PL, causing a flow of a thin film of liquid between the projection system PL and the substrate W. The choice of which combination of inlet IN and outlets OUT to use can depend on the direction of movement of the substrate W (the other combination of inlet IN and outlets OUT being inactive).

In European patent application publication no. EP 1420300 and United States patent application publication no. US 2004-0136494, each hereby incorporated in their entirety by reference, the idea of a twin or dual stage immersion lithography apparatus is disclosed. Such an apparatus is provided with two tables for supporting a substrate. Leveling measurements are carried out with a table at a first position, without immersion liquid, and exposure is carried out with a table at a second position, where immersion liquid is present. Alternatively, the apparatus has only one table.

SUMMARY

It is desirable to provide a liquid supply system in which the chance of gas being included in and/or liquid loss from the liquid reservoir positioned between the substrate and the projection system is reduced.

According to an aspect of the invention, there is provided a lithographic apparatus, comprising: a barrier member configured to surround a space between a projection system and a substrate and to at least partly constrain liquid in the space, the barrier member comprising an extractor configured to remove liquid from between the barrier member and the substrate, and a plate located between the extractor and the substrate such that a first channel, open to the space, is formed between the extractor and the plate and a second radially extending channel, open to the space, is formed between the plate and the substrate.

According to an aspect of the invention, there is provided a lithographic apparatus, comprising a liquid supply system configured to provide a liquid in a space between a projection system and a substrate, the liquid supply system including a structure configured to generate a seal between the liquid supply system and the substrate, the structure including a divider configured to divide a gap between the liquid supply system and the substrate into two such that, in use, the liquid forms two meniscuses, one above the divider and one below the divider.

According to an aspect of the invention, there is provided a lithographic apparatus, comprising a liquid supply system including a barrier member which surrounds a space between a projection system and a substrate and configured to form a contactless seal between the barrier member and the substrate; a liquid removal device configured to remove liquid and which surrounds the space and has an inlet which extends in the radial direction, the removal capacity of the liquid removal device being dependent on an amount of the inlet being covered by liquid; and a radially inwardly extending projection positioned between the inlet and the substrate to split the area between the inlet and the substrate in two.

According to another aspect, there is provided a lithographic apparatus, comprising: a barrier member configured to surround a space between a projection system and a substrate to at least partly constrain liquid in the space, the barrier member comprising a moveable surface and configured so that, in use, a meniscus of the liquid in the space extends between the surface and the substrate; and a controller adapted to control movement of the moveable surface in a direction with a component in the substantially same direction relative to the projection system as the substrate is moved and at a speed equal to two times the speed of the substrate or less during at least a portion of movement of the substrate.

According to another aspect, there is provided a lithographic apparatus, comprising a barrier member configured to surround a space between a projection system and a substrate and configured to at least partly constrain liquid in the space, the barrier member comprising a plate moveable in a plane substantially parallel to a top surface of the substrate and biased towards a neutral position and configured such that, in use, a meniscus of the liquid in the space extends between the plate and the substrate.

According to another aspect, there is provided a device manufacturing method, comprising: constraining a liquid in a space between a projection system and a substrate using a barrier member; moving the substrate relative to the projection system; moving a surface of the barrier member in a direction with a component in the substantially same direction as the movement of the substrate and at a speed equal to two times the speed of the substrate or less such that a meniscus of liquid between the surface and the substrate has forces on it reduced from what they would have been without movement of the surface during at least a portion of movement of the substrate; and transferring a pattern from a patterning device onto a substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

FIG. 1 depicts a lithographic apparatus according to an embodiment of the invention;

FIGS. 2 and 3 depict a liquid supply system for use in a lithographic projection apparatus;

FIG. 4 depicts another liquid supply system for use in a lithographic projection apparatus;

FIG. 5 illustrates, in cross-section, a further liquid supply system for use in a lithographic apparatus;

FIG. 6 illustrates, in cross-section, an embodiment of the present invention;

FIG. 7 illustrates, in cross-section, an embodiment of the present invention;

FIG. 8 depicts, in cross-section, an embodiment of the present invention;

FIG. 9 depicts, in cross-section, an embodiment of the present invention;

FIGS. 10 a-c depict schematically movements of a substrate W and a plate 200 under the projection system PL;

FIGS. 11 a-c illustrate schematically movements of the substrate W and plate 200 under the projection system PL;

FIG. 12 depicts, in cross-section, an embodiment of the present invention;

FIG. 13 depicts, in cross-section, an embodiment of the present invention;

FIG. 14 depicts, in cross-section, an embodiment of the present invention;

FIG. 15 also depicts, in cross-section, the embodiment of the present invention depicted in FIG. 14;

FIG. 16 depicts, in cross-section, an embodiment of the present invention; and

FIG. 17 also depicts, in cross-section, the embodiment of the present invention depicted in FIG. 16.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to one embodiment of the invention. The apparatus comprises:

an illumination system (illuminator) IL configured to condition a radiation beam B (e.g. UV radiation or DUV radiation);

a support structure (e.g. a mask table) MT constructed to support a patterning device (e.g. a mask) MA and connected to a first positioner PM configured to accurately position the patterning device in accordance with certain parameters;

a substrate table (e.g. a wafer table) WT constructed to hold a substrate (e.g. a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate in accordance with certain parameters; and

a projection system (e.g. a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.

The support structure holds the patterning device in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support structure can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support structure may be a frame or a table, for example, which may be fixed or movable as required. The support structure may ensure that the patterning device is at a desired position, for example with respect to the projection system. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning device.”

The term “patterning device” used herein should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate, for example if the pattern includes phase-shifting features or so called assist features. Generally, the pattern imparted to the radiation beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam which is reflected by the mirror matrix.

The term “projection system” used herein should be broadly interpreted as encompassing any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g. employing a transmissive mask). Alternatively, the apparatus may be of a reflective type (e.g. employing a programmable mirror array of a type as referred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more support structures). In such “multiple stage” machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from a radiation source SO. The source and the lithographic apparatus may be separate entities, for example when the source is an excimer laser. In such cases, the source is not considered to form part of the lithographic apparatus and the radiation beam is passed from the source SO to the illuminator IL with the aid of a beam delivery system BD comprising, for example, suitable directing mirrors and/or a beam expander. In other cases the source may be an integral part of the lithographic apparatus, for example when the source is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL may comprise various other components, such as an integrator IN and a condenser CO. The illuminator may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross-section.

The radiation beam B is incident on the patterning device (e.g., mask) MA, which is held on the support structure (e.g., mask table) MT, and is patterned by the patterning device. Having traversed the patterning device MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor IF (e.g. an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor (which is not explicitly depicted in FIG. 1) can be used to accurately position the patterning device MA with respect to the path of the radiation beam B, e.g. after mechanical retrieval from a mask library, or during a scan. In general, movement of the support structure MT may be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of the first positioner PM. Similarly, movement of the substrate table WT may be realized using a long-stroke module and a short-stroke module, which form part of the second positioner PW. In the case of a stepper (as opposed to a scanner) the support structure MT may be connected to a short-stroke actuator only, or may be fixed. Patterning device MA and substrate W may be aligned using patterning device alignment marks M1, M2 and substrate alignment marks P1, P2. Although the substrate alignment marks as illustrated occupy dedicated target portions, they may be located in spaces between target portions (these are known as scribe-lane alignment marks).

Similarly, in situations in which more than one die is provided on the patterning device MA, the patterning device alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the following modes:

1. In step mode, the support structure MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam is projected onto a target portion C at one time (i.e. a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed. In step mode, the maximum size of the exposure field limits the size of the target portion C imaged in a single static exposure.

2. In scan mode, the support structure MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e. a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS. In scan mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion in a single dynamic exposure, whereas the length of the scanning motion determines the height (in the scanning direction) of the target portion.

3. In another mode, the support structure MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above.

Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

Another immersion lithography solution with a localized liquid supply system solution which has been proposed is to provide the liquid supply system with a barrier member which extends along at least a part of a boundary of the space between the final element of the projection system and the substrate table. Such a solution is illustrated in FIG. 5. The barrier member is substantially stationary relative to the projection system in the XY plane though there may be some relative movement in the Z direction (in the direction of the optical axis).

The barrier member 12 at least partly contains liquid in the space 11 between a final element of the projection system PL and the substrate W. A contactless seal 16 to the substrate may be formed around the image field of the projection system so that liquid is confined within the space between the substrate surface and the final element of the projection system. The space is at least partly formed by the barrier member 12 positioned below and surrounding the final element of the projection system PL. Liquid is brought into the space below the projection system and within the barrier member 12 by liquid inlet 13 and may be removed by liquid outlet 13. The barrier member 12 may extend a little above the final element of the projection system and the liquid level rises above the final element so that a buffer of liquid is provided. The barrier member 12 has an inner periphery that at the upper end, in an embodiment, closely conforms to the shape of the projection system or the final element thereof and may, e.g., be round. At the bottom, the inner periphery closely conforms to the shape of the image field, e.g., rectangular though this need not be the case.

The liquid is contained in the space 11 by a gas seal 16 which, during use, is formed between the bottom of the barrier member 12 and the surface of the substrate W. The gas seal is formed by gas, e.g. air or synthetic air but, in an embodiment, N₂ or another inert gas, provided under pressure via inlet 15 to the gap between barrier member 12 and substrate and extracted via outlet 14. The overpressure on the gas inlet 15, vacuum level on the outlet 14 and geometry of the gap are arranged so that there is a high-velocity gas flow inwards that confines the liquid. Those inlets/outlets may be annular grooves which surround the space 11 and the flow of gas 16 is effective to contain the liquid in the space 11. Such a system is disclosed in United States patent application publication no. US 2004-0207824, hereby incorporated in its entirety by reference.

FIG. 6 illustrates a barrier member 10 which is part of a liquid supply system. The barrier member 10 extends around the periphery of the final element of the projection system PL such that the barrier member is, in an embodiment, substantially annular in overall shape. The projection system PL may not be circular and the outer edge of the barrier member 10 may also not be circular so that it is not necessary for the barrier member to be ring shaped and it could also be other shapes so long as it has a central opening through which the projection beam may pass out of the final element of the projection system PL through liquid contained in the central opening and onto the substrate W. The barrier member is described as being circular (this is an easy shape to manufacture) and references to radially (i.e. in and out towards the optical axis) are made. However, the term should be construed more generally to encompass movements with other geometries generally away from and towards the optical axis but in other geometries the directions may not exactly pass through the optical axis.

A function of the barrier member 10 is to maintain or confine liquid in the space between the projection system PL and the substrate W so that the projection beam may pass through the liquid. The top level of liquid is simply contained by the presence of the barrier member and the level of liquid in the space is maintained such that the liquid does not overflow over the top of the barrier member 10. In an embodiment, a seal is provided between the bottom of the barrier member 10 and the substrate W. In FIG. 6 the seal is a contactless seal and is made up of several components. Working radially outwardly from the optical axis of the projection system PL, there is provided a flow plate 50 which extends into the space (though not into the path of the projection beam) which helps maintain parallel flow of the immersion liquid out of inlet 20 across the space. The flow control plate has through holes 55 in it to reduce the resistance to movement in the direction of the optical axis of the barrier member 10 relative to the projection system PL and/or substrate W. Moving radially outwardly along the bottom of the barrier member 10 there is then provided an inlet 60 which provides a flow of liquid in a direction substantially parallel to the optical axis towards the substrate. This flow of liquid is used to help fill any gaps between the edge of the substrate W and the substrate table WT which supports the substrate. If this gap is not filled with liquid, bubbles may be included in the liquid in the space between the projection system PL and the substrate W when an edge of the substrate W crosses the seal. This is undesirable as it may lead to deterioration of image quality.

Radially outwardly of the inlet 60 is an extractor assembly 70 configured to extract liquid from between the barrier member 10 and the substrate W. The extractor 70 will be described in more detail below and forms part of the structure configured to create the contactless seal between the barrier member 10 and the substrate W.

Radially outwardly of the extractor assembly is a recess 80 which is connected through an inlet 82 to the atmosphere and via an outlet 84 to a low pressure source. Radially outwardly of the recess 80 is a gas knife 90. An arrangement of the extractor, recess and gas knife is disclosed in detail in U.S. patent application No. 60/643,626, filed 14 Jan. 2005, hereby incorporated in its entirety by reference. However, in that application the arrangement of the extractor assembly is different.

The extractor assembly 70 is comprised of a liquid removal device or extractor or outlet 100 such as the one disclosed in United States patent application publication US 2006-0038968, hereby incorporated in its entirety by reference. Any type of liquid extractor may be used. In an embodiment, the liquid removal device 100 comprises an outlet which is covered in a porous material 110 which is used to separate liquid from gas to enable single-liquid phase liquid extraction. A chamber 120 downstream of the porous material 110 is maintained at a slight under pressure and is filled with liquid. The under pressure in the chamber 120 is such that the meniscuses formed in the holes of the porous material prevent ambient gas (e.g., air) being drawn into the chamber 120 of the liquid removal device 100. However, when the porous material 110 comes into contact with liquid there is no meniscus to restrict flow and the liquid can flow freely into the chamber 120 of the liquid removal device 100. The porous material 110 extends radially inwardly along the barrier member 10 (as well as around the space) and its rate of extraction varies according to how much of the porous material 110 is covered by liquid.

Control of the meniscus of liquid between the barrier member 10 and the substrate W is desirable. During scanning of the substrate W (during which the substrate moves under the barrier member 10 and projection system PL) the meniscus may be drawn either towards or away from the optical axis by a drag force applied by the moving substrate W. This may lead to liquid loss which may result in evaporation of the liquid and thereby cooling of the substrate and consequent shrinkage and overlay errors. Alternatively or additionally, liquid stains may be left behind from interaction between liquid droplets and resist photochemistry. Another potential problem may be the inclusion of gas (e.g., air) into the space between the projection system PL and the substrate W which may lead to bubbles and deterioration in the quality of the projected image when the meniscus is dragged into the space. In an embodiment, a plate 200 is provided between the liquid removal device 100 and the substrate W so that the function of liquid extraction and the function of meniscus control can be separated from one another and the barrier member 10 may be optimized for each.

Viscous drag length, the distance between the start of the space (i.e. from the radially innermost edge of the flow control plate 50) and the meniscus between the barrier member 10 and the substrate W, may have a high influence on maximum scan speed possible without that meniscus collapsing. The accumulation of viscous drag forces may be reduced or minimized by keeping the viscous drag length low to reduce or minimize drag forces on the meniscus. However, the viscous drag length also generates a resistance which forces flow into an upper channel 220 (described below). Partly because of space constraints it may not be possible to arrange for the extractor assembly 70 to be any closer to the edge of the space. The height of the gap between the bottom of the barrier member 10 and the substrate W also affects the resistance. Thus, an optimum gap height and length may be determined by a balance between the resistance determined by the gap dimensions, viscous friction (smaller gap=bigger velocity gradient=bigger viscous friction), dynamic pressure (bigger gap=bigger amount of liquid to turn) and capillary pressure (bigger gap=smaller capillary pressure to withstand the load). In order to deal with these considerations, the plate 200 is positioned between the liquid removal device 100 and the substrate W. The plate 200 may be in the form of a projection which projects radially inwardly from a vertical surface of the barrier member 10 which is radially outwardly of the extractor assembly 70. The projection or plate 200 is a divider or any other element which has the function of splitting the space between the liquid removal device 100 and the substrate W into two channels, an upper channel 220 and a lower channel 230 wherein the upper channel 220 is between the upper surface of the plate 200 and the liquid removal device 100 and the lower channel 230 is between the lower surface of the plate 200 and the substrate W. Each channel is open, at its radially innermost end, to the space. The thickness of the plate is not critical. Although as illustrated in FIG. 6 the upper channel 220 extends horizontally, this is not necessarily the case. The reason for the upper channel 220 extending horizontally in FIG. 6 is because of the structural arrangement of the components. However, the upper channel 220 could also extend vertically or any where between horizontally and vertically. The gravitational pressure on the liquid in the upper channel 220 is very low and, if necessary, may be counteracted by applying an under pressure, for example through liquid removal device 100 itself or through another passage such as breathing holes 250 described below.

In an embodiment, the upper channel 220 between the liquid removal device 100 and the plate 200 is narrower than the lower channel 230 between the plate 200 and the substrate W. The lower channel is between 250 mm and 50 μm high, or between 100 and 60 μm depending on design (viscous drag length from flow pattern), fluid parameters (viscosity, density, surface tension) and/or surface properties (contact angle resulting from binding energy surface/liquid and liquid surface tension). The upper channel 220 has a stronger capillary action, for instance by making it 2 to 3 times narrower than the lower channel 230. Alternatively or additionally, the upper channel 220 may be made with a surface which is more liquid-philic than the lower channel. However, the upper channel 220 may also be wider than the lower channel 230. If the upper channel 220 is too narrow, liquid does not flow in that channel because the frictional resistance is too big and the meniscus between the plate 200 and the extractor 120 is fully loaded with hydrodynamic forces. Thus, if the upper channel 220 is made wider, for example in the region of 150 μm, than the lower channel 230 which could be perhaps 60 μm, these difficulties may be overcome. Above a channel width of 250 μm the capillary action is reduced. In order to promote capillary action, the upper channel 220 could be made liquid-philic or a height step close to the meniscus may be made such that the channel radially inwardly is wider than radially outwardly.

An under pressure may be applied in the upper channel 220, rather than leaving it open to the atmosphere through breathing holes 250 e.g. through the holes 250. In this way the upper channel 220 may be made wider.

Thus, there are two meniscuses 310, 320. A first meniscus 310 is positioned above the plate 200 and extends between the porous material 110 and the top surface of the plate 200 and a second meniscus 320 which is positioned underneath the plate 200 and which extends between the plate 200 and the substrate W. The extractor assembly 70 may be optimized to control the first meniscus for optimum extraction of liquid and to control the position of the second meniscus 320 such that the viscous drag length for the second meniscus is reduced. Also, the characteristics, in particular of the plate 200, of the extractor assembly 70 may be optimized to make it energetically favorable for the meniscus 320 to remain adhered to the plate 200 such that the scan speed of the substrate W beneath the barrier member 10 may be increased. Capillary forces acting on the second meniscus 320 are outwards and are balanced by an under pressure in the liquid adjacent the meniscus so that the meniscus may stay substantially still. Higher loading on the meniscus, for example by viscous drag and inertia, may be accommodated by a lowering of the contact angle of the meniscus with the surface.

In FIG. 6 the basic extractor assembly 70 is illustrated. Breathing holes 250 are provided at the radially outward most end of the plate 200 such that the first meniscus 310 is free to move inwardly and outwardly beneath the porous material 110 so that the extraction rate of the liquid removal device 100 may vary according to how much of the porous material 110 is covered by liquid. As illustrated in FIG. 6 the second meniscus 320 is adhered to a lower inner most edge of the plate 200.

In FIG. 6 the inner most bottom edge of the plate 200 is provided with a sharp edge so as to substantially pin the second meniscus 320 in place. The radius of the edge is, in an embodiment, less than 0.1 mm, less than 50 μm, less than 20 μm or about 10 μm.

An alternative way of substantially pinning the second meniscus 320 is to change the surface properties of the surface of the plate 200 to which the second meniscus 320 adheres. For example, a change from a liquid-philic to a liquid-phobic surface in a radially outward direction on the plate 200 could also result in pinning of the second meniscus 320 at that change because the shape of the second meniscus will need to invert for it to pass from the liquid-philic to the liquid-phobic surface. Another alternative is to change the surface of the plate 200 from a rough to a smooth surface. When fully wetted the rough surface can act as a meniscus trap. If the surface is not fully wetted and the liquid is only on the peaks of the roughness, a rough surface can act liquid-phobic such as in the so called lotus effect. Also electro wetting could be used to locally trap the meniscus. This has an advantage in that it can be turned on and off.

Two further embodiments are illustrated in FIGS. 7 and 8 which are useful for use in preventing the second meniscus 320 from moving outwards. In FIG. 7 the bottom surface of the plate 200 is angled such that its distance from the substrate W increases in an outward direction (away from the optical axis). Thus, the length of the second meniscus 320 will need to increase for the meniscus to move outwardly and this is energetically unfavorable. Conversely, the plate 200 has a top surface which is angled relative to the liquid removal device such that the distance between the top surface of the plate 200 and the liquid removal device 100 decreases further away from the optical axis. This encourages the first meniscus 310 to move outwardly such that the extraction capacity of the liquid removal device is increased.

Another or alternative way of pinning the second meniscus 320 substantially in place is to provide a step change in the distance between the bottom surface of the plate 200 and the substrate W such that in order to move past the step 280 the length of the second meniscus 320 will have to increase dramatically. Such an increase in length of the second meniscus 320 requires energy and is thus useful in pinning the position of the second meniscus 320.

FIG. 9 illustrates an embodiment which is the same as one or more of the embodiments described above except as described below. In this embodiment the plate 200 is translatable in a plane substantially parallel to the substrate W. When the substrate moves to the right hand side as illustrated by arrow 410, the plate 200 is also moved to the right hand side as indicated by arrow 400 at a speed equal to two times the speed of the substrate W or less. This is represented by the different sizes of arrows in FIG. 9. Of course both these movements are relative to the projection system PL and barrier member 10. In this relative speed range the speed difference between the substrate W and the plate 200 is reduced. For example if the plate is moved at half the velocity of the substrate W this means that the maximum scan speed at which the second meniscus 320 breaks is increased by a factor of 2 because the relative speed of the substrate to the plate is reduced by half. The first meniscus 310 only experiences the plate movement. It may be necessary to provide a breathing hole equivalent to breathing hole 250 in the side wall of the barrier member rather than through the plate 200. Lower speeds of the plate, say up to a speed equal to the substrate provide the same benefits as higher speeds up to twice the speed of the substrate but are easier to implement.

In practice the plate may move at any speed which reduces the relative velocity of the sealing surface of the plate 200 relative to the substrate W. During scanning in order to prevent cumulative movement in a certain direction the plate is only moved during the fast movements of the substrate W and may then be brought back gradually to a centered position during slower movement or during periods where there is no movement. Thus, during Y scans the plate may simply move up and down during the meandering and during X steps the plate may move with the substrate. During the Y scans the plate may move back to its original position to prevent accumulative movement in the X direction.

The movement of the plate may be either active or passive. In the case of an active plate a control system is provided which interacts with data from the overall controller of the lithographic apparatus to coordinate movements of the plate 200 with movements of the substrate W. The plate 200 may be actuated by, for example, a piezoelectric actuator, a linear motor, etc. In an embodiment in which the plate is moved passively, the plate may be attached to the barrier member 10 by springs positioning it in the X-Y direction. Friction between the moveable plate 200 and the substrate W will provide enough force for the plate to be moved in the same direction as the substrate W. By adding a liquid phobic coating at an edge of the plate 200, the force exerted on the plate by the substrate W through the liquid is increased. The strength of the springs is chosen-such that the plate moves only during step movements of high speed.

FIGS. 10 and 11 show two practical examples of the moving plate described above in which the movement of the plate is active i.e. it is moved with actuators. FIG. 10 a shows the movement of the substrate W under the projection system PL and FIG. 10 b shows the movement of the plate 200 under the stationary barrier member 10 and projection system PL. The substrate is scanned under the projection system PL from position 1 to position 2 (FIG. 10 a) such that the shaded square box is illuminated as the patterning device is scanned across. During this scanning, the plate 200 follows (slowly) a diagonal path from position 1 to position 2 (FIG. 10 b). During the step motion (2-3-4), to reduce the relative speed of the plate 200 and substrate W during the step motion, the plate can then also move from left to right as illustrated (2-3-4) in FIG. 10 b. During the scan from 4-5 another diagonal path is traversed slowly by the plate 200 in order that the plate is in a position at the end of that scan to move once again in the X direction to reduce the relative speed of the substrate to the plate during the X step. FIG. 10 c illustrates the absolute speeds of the substrate W and the plate 200 as well as the relative speed of the substrate W to the plate 200. From this graph it is easy to see how the moveable plate embodiment reduces the relative speed of the substrate W relative to the plate 200 and thereby allows greater speed of the substrate W leading to increased throughput before meniscus breakdown.

FIGS. 11 a-c illustrate a similar principle to that illustrated in FIGS. 10 a-c except for longer scans to allow for double exposure.

Although this embodiment is described in relation to the case where a meniscus is present above and below the plate, this is not necessarily the case and this principle can be used for other applications particularly in the immersion lithography field. In an alternative embodiment the plate 200 is dispensed with and the bottom surface of the barrier member 10 is used in the same way as the plate is described above. If a larger central opening is provided in the barrier member 10 through which the projection beam PB can be projected onto the substrate W, the barrier member 10 may itself be moved in the same direction as the substrate W in order to reduce the relative velocity of the substrate W relative to the sealing surface of the barrier member 10.

In an embodiment, a controller may predict likely movements required by the sealing surface in the future because it has the data regarding the movement which the substrate will undergo during exposure so that it can, during slow movement of the substrate relative to the projection system move the moveable plate/member close to an extreme position (or back to a central position) so as to maximize the available movement of the plate/member for a future high speed movement of the substrate relative to the projection system.

FIG. 12 illustrates an embodiment which is the same one or more of the embodiments above except as described below. In this embodiment and the embodiment described below with respect to FIG. 13, a measure is applied to reduce the hydrodynamic load of the flow profile behind the second meniscus 320 between the plate member 200 and the substrate W. A counter current flow is imposed in the liquid between the plate 200 and the substrate W in the vicinity of the plate 200 to generate a vortex behind the meniscus. This vortex assists in lifting the meniscus from the substrate W. As a result, the chance of meniscus collapse is reduced or minimized.

The vortex (illustrated by arrow 505) is induced by providing one or more outlet ports 500 in a bottom face of the plate, divider, projection or moveable sealing surface 200 which faces the substrate W. The outlet port(s) 500 is maintained at an under pressure by a low pressure source at a pressure of 0.5 to 1.5 atm, or between 0.5 and 1.0 atm, lower than that of the liquid in the space such that liquid is sucked out of the space between the plate member 200 and the substrate W (as illustrated with arrow 515). Thus, compared to one or more of the embodiments described above, the second meniscus 320 is positioned further radially outwardly. The meniscus is pinned substantially in place by a meniscus pinning feature 510 which may take any form, for instance such as those described above. In an embodiment, feature 510 is a sharp edge with a radius of about 10 μm.

In FIG. 13, an embodiment is illustrated which is the same as the embodiment described with respect to FIG. 12 except that the outlet port(s) 500 of FIG. 12 is replaced with one or more inlet ports 600. Jets of liquid are supplied through the inlet port(s) 600 and induce the vortex 505 which reduces or minimizes the chance of meniscus collapse. The inlet port(s) 600 is angled with a component radially inwardly as well as vertically downwardly towards the substrate W such that there is a component of the liquid entering the space radially inwardly. A range of angles of between 10 and 60 degrees off vertical radially inwardly may be suitable.

In the embodiments described above, the vortex 505 which is generated is a vortex which is used to stabilize a receding meniscus. A meniscus is receding if the substrate W is moving under the barrier member 10 at the position of the meniscus radially outwardly (i.e. dragging the bottom part of the meniscus radially outwardly beyond the meniscus pinning feature 510). In the case of an advancing meniscus (in which the bottom part of the meniscus attached to the substrate W is being dragged more towards under the projection system PL), it may be beneficial to reverse the direction of the vortex 505 by either expelling immersion liquid through outlet port(s) 500 or by sucking immersion liquid through inlet port(s) 600.

In FIG. 14, an embodiment is illustrated which is the same as one or more of the embodiments described above except as outlined below. In FIG. 14 only the plate 200 and the substrate W are illustrated. This is because only a different meniscus pinning feature 510 will be discussed. This meniscus pinning feature may be used in any of the embodiments described herein.

In FIG. 14 a gas inlet and/or outlet port 700 is provided at the edge 710 where the second meniscus 320 is to be substantially pinned. In the situation illustrated in FIG. 14, the substrate W is moving from right to left in the Figure relative to the plate 200. Thus, a dragging force is felt by the meniscus (a so called receding meniscus) due to frictional forces with the surface of the substrate W. In order to avoid meniscus breakdown, a flow of gas 750 is created by applying an under pressure (e.g., about 10 mbar) to the in/out-let port 700. This flow of gas 750 is radially inwardly and impinges against the meniscus 320 and is then directed with a vertical component into the in/out-let port 700. The frictional force on the meniscus 320 exerts a drag force on the meniscus that stabilizes the meniscus location. As a result, the speed at which the meniscus breaks down is increased significantly. Thus, the scan speed may be greatly increased.

FIG. 15 shows an embodiment where the meniscus is advancing. In other words, the substrate W is moving from left to right in the Figure relative to the plate 200 thereby inducing a force on the meniscus 320 which tries to drag the meniscus away from the edge 710 and the in/out-let port 700. In this instance an over pressure (e.g., 10 mbar) may be applied to the in/out-let port 700 such that a flow of gas 760 downwards and then radially outwardly away from the meniscus 320 is created. This may also help to stabilize the meniscus when the meniscus is advancing.

A controller may be provided to control when an over or an under pressure is applied to the in/out-let port 700. This controller could receive as input, for example, control signals which are also sent to a positioner used to position the substrate W under the projection system PL. Based on this information the controller could compute whether or not the meniscus at any point around the periphery of the barrier member 10 will be locally advancing or receding and apply an under or over pressure at the local area to the in/out-let port as required in order to stabilize the meniscus.

FIGS. 16 and 17 show an embodiment which is the same as one or more of the embodiments described above except as outlined below. In FIGS. 16 and 17 only the plate 200 and the substrate W are illustrated. This is because only a different meniscus pinning feature 510 will be discussed. This meniscus pinning feature may be used in any of the embodiments described herein.

In this embodiment, passive means are used to pin the position of the meniscus and two of the previously mentioned techniques are used in combination. FIG. 16 shows the situation with a receding meniscus and FIG. 17 shows the situation with an advancing meniscus. Along the bottom of the plate 200 is a step 800 so that the outer bottom of the plate 200 is lower (i.e. closer to the substrate W) than the radially inward portion of the bottom surface of the plate 200.

On the bottom surface of the plate 200 radially outwardly of the edge 800 is an area which is made liquid-phobic meaning that the contact angle of the immersion liquid with the surface is greater than 45°, 70°, 80°, 90°, 100°, 110° or 120°. Thus, in the receding situation shown in FIG. 16 it is the combination of the liquid-phobic area 810 and the edge 800 which pins the meniscus to the plate 200 and in the advancing situation shown in FIG. 17 it is mainly the edge 800 which pins the position of the meniscus.

As appropriate, one or more features of one or more embodiments described herein may be added to, used alternatively or combined in one or more of the other embodiments described herein.

Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion”, respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention may be used in other applications, for example imprint lithography, and where the context allows, is not limited to optical lithography. In imprint lithography a topography in a patterning device defines the pattern created on a substrate. The topography of the patterning device may be pressed into a layer of resist supplied to the substrate whereupon the resist is cured by applying electromagnetic radiation, heat, pressure or a combination thereof. The patterning device is moved out of the resist leaving a pattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g. having a wavelength of or about 365, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range of 5-20 nm), as well as particle beams, such as ion beams or electron beams.

The term “lens”, where the context allows, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components.

While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. For example, the invention may take the form of a computer program containing one or more sequences of machine-readable instructions describing a method as disclosed above, or a data storage medium (e.g. semiconductor memory, magnetic or optical disk) having such a computer program stored therein.

One or more embodiments of the invention may be applied to any immersion lithography apparatus, in particular, but not exclusively, those types mentioned above and whether the immersion liquid is provided in the form of a bath or only on a localized surface area of the substrate. A liquid supply system as contemplated herein should be broadly construed. In certain embodiments, it may be a mechanism or combination of structures that provides a liquid to a space between the projection system and the substrate and/or substrate table. It may comprise a combination of one or more structures, one or more liquid inlets, one or more gas inlets, one or more gas outlets, and/or one or more liquid outlets that provide liquid to the space. In an embodiment, a surface of the space may be a portion of the substrate and/or substrate table, or a surface of the space may completely cover a surface of the substrate and/or substrate table, or the space may envelop the substrate and/or substrate table. The liquid supply system may optionally further include one or more elements to control the position, quantity, quality, shape, flow rate or any other features of the liquid.

The immersion liquid used in the apparatus may have different compositions, according to the desired properties and the wavelength of exposure radiation used. For an exposure wavelength of 193 nm, ultra pure water or water-based compositions may be used and for this reason the immersion liquid is sometimes referred to as water and water-related terms such as hydrophilic, hydrophobic, humidity, etc. may be used.

The descriptions above are intended to be illustrative, not limiting. Thus, it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below. 

1. A lithographic apparatus, comprising: a barrier member configured to surround a space between a projection system and a substrate and to at least partly constrain liquid in the space, the barrier member comprising: an extractor configured to remove liquid from between the barrier member and the substrate, and a plate located between the extractor and the substrate such that a first channel, open to the space and to the extractor, is formed between the extractor and the plate and a second radially extending channel, open to the space, is formed between the plate and the substrate, wherein the first channel has, in use, an under pressure other than from the extractor or has, in use, an atmospheric pressure, and wherein, in use, liquid flows into the first channel to form a meniscus extending from the extractor in the channel.
 2. The apparatus of claim 1, wherein the first channel is radially extending.
 3. The apparatus of claim 1, wherein the first channel is narrower than the second channel.
 4. The apparatus of claim 1, wherein the first channel narrows in a radially outward direction.
 5. The apparatus of claim 1, wherein the second channel widens in a radially outward direction.
 6. The apparatus of claim 1, wherein the plate has a meniscus pinning feature to make it energetically unfavorable for a meniscus attached to the feature to move radially outwardly of the feature.
 7. The apparatus of claim 6, wherein the feature includes a port configured to be connected to an under pressure source, an over pressure source, or both, to generate a gas flow over the meniscus attached to the feature.
 8. The apparatus of claim 7, wherein the port extends around the whole of the barrier member.
 9. The apparatus of claim 8, wherein the port comprises a plurality of individually controllable sections such that a part of the port can be exposed to the under pressure while another part can be exposed to the over pressure.
 10. The apparatus of claim 1, wherein the bottom surface of the plate has a step formed in it such that the second channel is wider radially outward of the step.
 11. The apparatus of claim 1, wherein the bottom surface of the plate has a step formed in it such that the second channel is wider radially inward of the step.
 12. The apparatus of claim 11, configured such that the liquid would have a contact angle with the bottom surface radially outward of the step of more than 45°.
 13. The apparatus of claim 1, wherein the plate has a radially inner most bottom edge which has a radius of less than 0.1 mm.
 14. The apparatus of claim 1, wherein the plate has a change in surface property on an under side from liquid-phobic to liquid-philic in a direction radially outwardly.
 15. The apparatus of claim 1, wherein the plate has a change in surface roughness from relatively rough to relatively smooth on an underside in a direction radially outwardly.
 16. The apparatus of claim 1, wherein the extractor comprises a porous material through which the liquid is to be removed.
 17. The apparatus of claim 16, wherein the porous material extends in a radial direction and an extraction capacity of the extractor is configured to increase with increasing area of the porous material covered by liquid during use.
 18. The apparatus of claim 1, wherein the plate is moveable in a plane substantially parallel to a top surface of the substrate.
 19. The apparatus of claim 18, further comprising a controller adapted to control the movement of the plate in substantially the same direction relative to the projection system as the substrate is moved and at a speed equal to two times the speed of the substrate or less during at least a portion of movement of the substrate.
 20. The apparatus of claim 19, wherein the controller is further adapted to move the moveable plate in the substantially same direction as the substrate during a highest speed movement of the substrate.
 21. The apparatus of claim 19, wherein the controller is further adapted to move the moveable plate in a direction against the movement of the substrate during at least a portion of a low speed movement of the substrate in the direction or to move the moveable plate in a direction in which the substrate is substantially stationary during a portion of time in which the substrate is substantially stationary in the direction.
 22. The apparatus of claim 18, wherein the moveable plate is moveable relative to the barrier member.
 23. The apparatus of claim 1, further comprising an inlet, an outlet, or both, in a face of the plate configured to face the substrate.
 24. The apparatus of claim 23, wherein the inlet, the outlet, or both, is configured to be connected to a low pressure source, a high pressure source, or both, to generate a vortex between the plate and the substrate.
 25. The apparatus of claim 24, configured to provide, to the inlet, the outlet, or both, a pressure of between 0.5 and 1.0 atm below that of liquid between the plate and the substrate.
 26. The apparatus of claim 23, further comprising a meniscus pinning feature on the face of the plate and positioned radially outwardly of the inlet, the outlet, or both.
 27. The apparatus of claim 1, further comprising an inlet in a face of the plate configured to face the substrate.
 28. The apparatus of claim 27, wherein the inlet is shaped to generate a flow of liquid at least partly radially inwardly to generate a vortex in liquid between the plate and the substrate that would be radially outwardly nearer the substrate, vertically towards the plate and then radially inwardly along the plate.
 29. The apparatus of claim 27, further comprising a meniscus pinning feature on the face of the plate and positioned radially outwardly of the inlet.
 30. The apparatus of claim 27, wherein inlet is configured to generate a jet to generate a counter flow of liquid between the plate and the substrate.
 31. The apparatus of claim 1, further comprising a port in the plate configured to generate a vortex in liquid between the plate and the substrate.
 32. The apparatus of claim 1, wherein the first channel is connected to an under pressure source to generate an under pressure in the first channel.
 33. The apparatus of claim 1, wherein the plate is constructed and arranged to make it energetically favorable for a meniscus below the plate to be positioned attached to a radially inner most edge of the plate.
 34. The lithographic apparatus of claim 1, wherein the plate is moveable in a plane substantially parallel to a top surface of the substrate and biased towards a neutral position and the barrier member is configured such that, in use, a meniscus of the liquid in the space extends between the plate and the substrate.
 35. The apparatus of claim 34, wherein the biasing is such that forces transmitted through the liquid from the substrate to the plate are high enough during high speed movement of the substrate to move the plate relative to the barrier member.
 36. A lithographic apparatus, comprising: a liquid supply system including a barrier member which surrounds a space between a projection system and a substrate and configured to form a contactless seal between the barrier member and the substrate; a liquid removal device configured to remove liquid and which surrounds the space and has an inlet which extends in the radial direction, the removal capacity of the liquid removal device being dependent on an amount of the inlet being covered by liquid; and a radially inwardly extending projection positioned between the inlet and the substrate to split the area between the inlet and the substrate in two, wherein, in use, a meniscus extends from the inlet in the area between the inlet and the projection.
 37. The apparatus of claim 36, wherein the area between the inlet and the projection narrows in a radially outward direction or the area between the projection and the substrate widens in a radially outward direction.
 38. The apparatus of claim 36, wherein the projection has a meniscus pinning feature to make it energetically unfavorable for a meniscus attached to the feature to move radially outwardly of the feature.
 39. The apparatus of claim 38, wherein the feature includes a port configured to be connected to an under pressure source, an over pressure source, or both, to generate a gas flow over the meniscus attached to the feature.
 40. The apparatus of claim 36, further comprising an inlet, an outlet, or both, in a face of the projection configured to face the substrate.
 41. The apparatus of claim 40, wherein the inlet, the outlet, or both, is configured to be connected to a low pressure source, a high pressure source, or both, to generate a vortex between the projection and the substrate.
 42. The apparatus of claim 40, further comprising a meniscus pinning feature on the face of the projection and positioned radially outwardly of the inlet, the outlet, or both.
 43. The apparatus of claim 40, wherein the projection comprises the inlet and wherein the inlet is shaped to generate a flow of liquid at least partly radially inwardly to generate a vortex in liquid between the projection and the substrate that would be radially outwardly nearer the substrate, vertically towards the projection and then radially inwardly along the projection.
 44. The apparatus of claim 40, wherein the projection comprises the inlet and wherein inlet is configured to generate a jet to generate a counter flow of liquid between the projection and the substrate. 